Some features (e.g., override-hints and clause-processors) of
serial ACL2 are by default not available in ACL2(p) with waterfall
parallelism enabled, because they offer a mechanism to modify state that
is unsound. To allow or (once again) disallow the use the these features in
ACL2(p), call set-waterfall-parallelism-hacks-enabled with argument t
or nil, respectively.